Throughout this book, we have discussed the value of the conventional surface ECG for the diagnosis of heart diseases in different situations. We have emphasized the need for a comprehensive approach to diagnosis, to always perform the interpretation of the surface ECG in light of the patient’s clinical setting. This approach is essential and sufficient in many cases, but in other circumstances, as demonstrated, it may not provide the necessary information for the correct diagnosis with the highest sensitivity, specificity, and overall predictive value. Furthermore, it is often not sufficient for risk stratification and for arriving at therapeutic decisions. For that, the use of other diagnostic techniques may be necessary. We will now make some comments about the importance of the clinical context for ECG interpretation, and later we will briefly describe the utility of other electrocardiologic techniques. As discussed throughout the text, the following examples illustrate the need to use information related to the patient’s clinical setting for the interpretation of the ECG: The ECG can be normal even just before the patient experiences sudden death. This can occur in the following circumstances: We will briefly discuss different techniques that may help us when the information derived from the clinical setting and conventional ECG are not sufficient to arrive at correct diagnosis, projecting the prognosis and/or developing a therapeutic approach in each case. We will assume that the reader is familiar with the most important features of each technique. However, we will give now a brief discussion on the most widely used. We made some comments about computerized ECG interpretation systems in Chapters 3 and 10. More information on that is beyond the scope of this book. With regard to the ECG classification systems, the most widely known are the Minnesota Code (MC) (Figure 25.1) and the Nova code (Nova). Both are valuable and useful for clinical trials and epidemiological studies. For more information about computers and classification systems, please consult Macfarlane (2010). Vectorcardiography (VCG) is a technique that records the cardiac electrical activity as closed loops: the atrial depolarization (P loop), ventricular depolarization (QRS loop), and ventricular repolarization (T loop) loops. VCG curves originate from X, Y, and Z leads (Figure 25.2A), which are three orthogonal leads that are perpendicular to each other. The X lead is right–left (similar to lead 1), Y is supero‐inferior (similar to aVF), and Z is postero‐anterior (similar to V2). These leads are generally recorded with the Frank system by means of different electrodes located on various points of the body. Inscription of the curves is done with a preamplifier system that magnifies the voltage of the currents coming from the heart through the lead systems, and a cathodic X‐ray tube by which the vectorcardiographic curve is seen. VCG machines interrupt the current every 5 ms, 2.5 ms, or 1 ms using an oscillator; the VCG continuous curves are thus divided into tears or comma‐shaped fragments, the head of which represents the direction of the electrical current (Figure 25.2B). Figure 25.2C shows the ECG morphology that corresponds to the VCG of Figure 25.2B. We draw the loops as continuous lines when we use the VCG curves to aid in the comprehension of the ECG. Figure 25.1 An ECG interpretation using the Minnesota Code: 1.1. 7 (sign of necrosis); 2.1, frontal plane axis −30°, 5.1, repolarization disorder; negative T wave; 8.8, sinus bradycardia. In this case, our interpretation would be: sinus bradycardia, old anteroseptal myocardial infarction with inferior extension, anterolateral subepicardial ischemia, and mild inferolateral subepicardial injury. Figure 25.2 (A) Orthogonal leads with the corresponding vectorcardiographic (VCG) loop and its projection on the frontal, horizontal, and right sagittal planes. (B) Normal VCG corresponding to ECG of (C). At a sensitivity of 4, the P and T loops are poorly seen and the entire QRS loop is clearly visible (upper part of B). In the middle and lower panels of (B), the P and T loops with the respective onset and end of the QRS loop with amplified sensitivity may be seen. Vectorcardiography is useful both as a clinical and teaching tool, especially for training in electrocardiography. Electrocardiography should be integrated with VCG, as described in this text; one should be able to deduce ECG morphology from the VCG curve, and vice versa. Table 25.1 shows the classical usefulness of VCG (Benchimol et al. 1972). However, currently the clinical utility of vectorcardiography has progressively decreased, and as we have explained in Chapter 3 with the correlation of the morphology of the loops with the hemifields and ECG curves, most of the advantages of VCG loops may be found in the surface ECG. In recent years, the possible usefulness of spatial QRS–T angle for risk stratification and some other utilities of VCG have been described. However, the most important contribution of VCG nowadays is its use, as we have done in this book, for teaching purposes (see Chapter 3). Figure 25.3 Normal morphologies, rotation, and orientation of the P, QRS, and T loops in three planes: frontal (A), horizontal (B), and sagittal (C). The QRS loop in the frontal plane may have clockwise rotation with an upward initiation (sometimes it forms a figure‐of‐eight) or, less often, counterclockwise rotation with an inferior onset. The latter is seen especially in obese subjects. EO = P loop; OJ = QRS loop; JE = T loop. The onset of the ST vector begins at point E, and the termination, at point J. Table 25.1 Classical clinical usefulness of vectorcardiography (see text) Exercise is considered isotonic or dynamic when several muscle groups alternately contract and relax, as in running, and isometric or static, when few muscle groups contract for more prolonged periods against a fixed force, as in weightlifting. Isotonic exercise such as that on a bicycle or treadmill is the most appropriate form of exercise to assess cardiac functional capacity. Exercise testing represents more than the establishment of ECG changes during exercise. Hemodynamic or metabolic (O2 consumption, quantity of exercise, changes in blood pressure, and heart rate, etc.) and clinica1 changes (presence of anginal pain, dyspnea, etc.) should also be evaluated. Figure 25.4 Six examples of exercise ECG–thallium scintigraphy correlation (S = exercise image, R = redistribution image). (A) Positive exercise testing in an asymptomatic patient with negative thallium and normal coronary angiography. (B) Patient with exercise angina, with a negative exercise test and stress images showing a mild defect in the lower septum, with complete redistribution. (C) A patient with an inferolateral infarction with a positive exercise test and exercise thallium image showing an inferolateral defect without redistribution. (D) Patient with angina, with a positive exercise test from V2 to V6 and 1, 11, VL, and thallium images showing an inferolateral defect with complete redistribution. (E) Patient with anterior myocardial infarction, without significant change in exercise testing. The thallium images show the existence of an anteroseptal defect without redistribution and marked dilatation of the left ventricle. (F) Patient with inferolateral myocardial infarction, with a positive exercise test in the lateral leads. Thallium images show an inferoposterolateral defect with only lateral redistribution (positive for inferoposterior necrosis and lateral ischemia). The bicycle and treadmill are equally useful for exercise testing. There is no ideal protocol, but the Bruce protocol is the most widely used (Table 25.2). The following general principles should be taken into account: (i) The intensity of exercise should be increased gradually, not suddenly. Increments are generally made at a minimal interval of 3 minutes. (ii) Patients should be monitored for symptoms (precordial pain, etc.), ECG changes, and hemodynamic changes (blood pressure and heart rate) during the exercise and for at least 6–8 minutes after the test. (iii) Exercise should not be stopped abruptly. Exercise capacity is described using the product of heart rate and blood pressure, which is the so‐called double product. A submaximal exercise test (85–90% of theoretical maximum heart rate for the patient’s age and sex) is adequate for clinical purposes and is much easier to perform for patients with ischemic heart disease. Metabolic equivalents (METS) (multiples of basal metabolic requirements) are used to express the work performed at different stages of the exercise test. In patients with ischemic heart disease, a workload of 8 METS is usually sufficient to evaluate angina. Healthy sedentary individuals do not usually exceed 10–11 METS, while athletes usually achieve more than 16 METS. The exercise testing should be interrupted when: (i) significant symptoms or arrhythmias appear, (ii) significantly abnormal ST segment changes are detected, or (iii) the target heart rate is reached. Figure 25.5 Diagnosis of ischemic heart disease by correlating clinical data (upper left) with exercise test results. Counterclockwise from the upper left, presence or absence of chest pain on the treadmill (lower left) (B); positive exercise ECG test with ST depression (ECG‐ST) (lower right) (C); or positive thallium imaging (upper right) (D). Shaded curves indicate the mean (± standard deviation) for 96 patients. Different shaded curves represent positive (+) or negative (−) results; non‐diagnostic ECG–ST results are shown by a question mark. For clinical data (upper left), the age and sex are shown on the vertical axis versus the probability of ischemic heart disease on the horizontal axis. Separate curves are shown according to the number of risk factors (0, 1–2, or 3–5) for asymptomatic patients (men and women). Symptomatic patients are classified as having no angina (NACP), atypical chest pain (ATCP), or typical angina pectoris (TAP). The post‐test probability of ischemic heart disease according to each test becomes the pretest probability of the next test in sequence, moving counterclockwise. The lines represent the range of probabilities of ischemic heart disease (IHD) for two patients. Patient 1 is a 45‐year‐old man with no typical symptoms but three risk factors, and patient 2 is 45‐year‐old man with chest pain typical of angina pectoris. (Reproduced with permission Patterson et al. 1984). Table 25.2 Bruce protocol for exercise (treadmill) ECG test (Based on Benchimol et al. 1972). METS: metabolic equivalents. Exercise testing is very useful in patients with ischemic heart disease to arrive at the diagnosis, to evaluate functional capacity, and to monitor the response to treatment. It can also be useful in other heart diseases and in the evaluation and assessment of cardiac arrhythmias. The most important indications and contraindications of exercise testing are listed in Tables 25.3 and 25.4. The most important indication is to determine the presence of ischemia (positive test) in patients with dubious precordial pain and in post‐infarction patients to stratify prognosis. The combination of the appearance of anginal pain or other clinical or hemodynamic signs (Table 25.5) and electrocardiographic ST depression confirms the diagnosis. If it is only electrocardiographically positive, the diagnosis can only be suggested, although other tests (another exercise test with isotopic methods and on occasions, coronary angiography) are needed to clarify the problem and to exclude false positive results. The degree of abnormality of the exercise test is very important. If it is clearly positive (early and/or important ST depression, and/or precordial pain or hypotension, etc.), coronary angiography is recommended. If it is equivocal for ischemia (minor ST depression at the final stage of the Bruce protocol) without severe clinical findings (angina, hypotension), we recommended other techniques (isotopic or CV), magnetic resonance studies of perfusion, to assure the diagnosis of ischemia. If these tests are positive for ischemia, coronary angiography or multislice scanner is recommended. Table 25.3 Main indications for exercise testing Table 25.4 Contraindications for exercise testing
Chapter 25
Limitations of the Conventional ECG: Utility of Other Techniques
Introduction
Interpretation of the surface ECG in light of the patient’s clinical setting
The abnormal ECG in the absence of heart disease and patients with normal ECG and advance heart disease (Bayés de Luna et al. 2020a)
Patient with advanced heart disease and normal ECG
Patients without heart disease and with abnormal ECG
Additional value of other techniques
Unified interpretation of the ECG: computerized interpretation and the use of ECG classification systems
Vectorcardiography (Figures 25.2 and 25.3)
Characteristics of different loops
Usefulness of vectorcardiography
Exercise testing (Figures 25.4 and 25.5)
Methodology
Stage
Speed
(mph)
Grade
(%)
Duration
(min)
METS
(units)
Total time (min)
1
1.7
10
3
4
3
2
2.5
12
3
6–7
6
3
3.4
14
3
8–9
9
4
4.2
16
3
15–16
12
5
5.0
18
3
21
15
6
5.5
20
3
—
—
7
6.0
22
3
—
—
Usefulness
Diagnostic indications
Doubtful precordial pain. Early detection of ischemic heart disease
Functional
Arrhythmias and exercise
Prognosis in patients with ischemic heart disease (post‐myocardial infarction patients)
Severity of ischemic heart disease
Functional capacity of patients with heart disease
Therapeutic effectiveness
Behavior, with exercise, of known arrhythmias
Level of physical training in asymptomatic patients and athletes
Other indications
Rehabilitation
Research
Absolute
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Limitations of the Conventional ECG
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